In a compressor which is employed as an element of a refrigeration cycle for a refrigerator, oil is supplied for the purpose of lubricating a bearing part of a motor driving shaft and a compression element, and this oil is mixed into a refrigerant in the compressor, passes through the compression element with the refrigerant and is discharged into a high-pressure chamber. When the oil thus mixed into the refrigerant is discharged from the compressor and enters the refrigeration cycle, the heat exchange rate reduces in heat exchange in an evaporator forming the refrigeration cycle. Therefore, means for separating the oil entering the high-pressure chamber in the compressor from the refrigerant and returning the same to the low-pressure chamber is adopted. An oil separator for a compressor to which the present invention is directed is provided as one of such means, for the purpose of separating oil entering a high-pressure chamber in the compressor from a refrigerant.
As the prior art showing this type of oil separator for a compressor, a scroll compressor on which an oil separator including a demister is mounted, disclosed in Japanese Patent Laying-Open No. 1-240787, is mentioned. The scroll compressor disclosed in this gazette connects a casing top 3 opening an outer discharge port 2 to an upper end portion of a casing body 1, and press-fits an end plate 5 of a fixed scroll 4 into an opening side end portion of this casing top 3 by shrinkage fitting or the like, for forming a discharge chamber 6 which is divided from the casing body 1 above the end plate 5, as shown in FIG. 6.
A frame 8 is supported for the fixed scroll 4 in opposition to a movable scroll 7, while a motor (not shown) is provided under the frame 8, so that the movable scroll 7 is revolved/driven with respect to the fixed scroll 4 by driving of this motor. Refrigerant gas which is inhaled from a low-pressure chamber 10 is compressed between the fixed-movable scrolls 4 and 7 by this revolution/driving of the movable scroll 7, so that this compressed refrigerant gas is discharged from a discharge hole 11 provided on a central portion of the fixed scroll 4 into the discharge chamber 6, and discharged into a discharge pipe 12 through the outer discharge port 2.
An oil separator 13 is mounted on the upper surface of the fixed scroll 4 of this scroll compressor, so that the oil which is discharged into the discharge chamber 6 with the refrigerant gas is separated from the refrigerant gas by this oil separator 13. The separated oil is returned to the bottom side of the casing body 1 through an oil return tube 14 extending in the vertical direction through the fixed scroll 4 and the frame 8. A concave part 15 is formed around the discharge chamber 6 side end portion of the oil return tube 14, for collecting the oil separated from the refrigerant gas in this concave part 15 and precipitating dust out of the oil.
The oil separator 13 is formed by a netted demister 13a which is annularly formed by winding a metal thin wire of stainless steel or the like or a woven strip of a plurality of stranded metal thin wires a prescribed number of times, and a plate-type fitting 13b which is integrated with the demister 13a to cover a part of its periphery and its upper surface.
As lubricating oil for a compressor for a refrigerator, SUNISO 4GS based on mineral oil has been mainly employed since its affinity with a flon-based R22 refrigerant generally employed as a refrigerant for a refrigerator is excellent. This SUNISO 4GS is the so-called nonpolar oil, which has no polarity since no oxygen group is provided in its molecular structure. The nonpolar oil hardly forms monomolecular films on the surfaces of the demister 13a of the oil separator 13, and hence the wettability of the oil with respect to the oil separator 13 is not damaged and the oil capturability by the oil separator 13 is excellently maintained.
In recent years, however, destruction of the ozone layer with the flon-based refrigerant comes into question, and there is a tendency to replace the refrigerant with R407C, which is an HFC (Hydro-Fluoro-Carbon) refrigerant destructing no ozone layer since it contains no chlorine, in response to the international trend of regulating employment of the flon-based refrigerant. As lubricating oil in case of employing this HFC refrigerant, ester or ether oil which is chemosynthetic oil having excellent affinity with the HFC refrigerant is employed. In such lubricating oil, ester or ether oil has polarity since the same has an oxygen group in its molecular structure. Such polar oil readily forms monomolecular films on the surfaces of the demister 13a of the oil separator 13, and once monomolecular films are formed on the surfaces of the demister 13a, the wettability of the oil with respect to the oil separator 13 thereafter deteriorates. In other words, the surfaces of the demister 13a provided with the monomolecular film readily repel the oil, and hence the oil is hardly captured by the oil separator 13 and readily re-scattered. Consequently, the oil discharged into a high-pressure chamber is not separated from the refrigerant but enters the refrigeration cycle to reduce the heat exchange rate in an evaporator or the like, and hence the refrigeration efficiency is disadvantageously reduced.